Boosting LED driver

If you need the power (V & I), you should build it out of discrete components. Power transistors need room to cool and Inductors and capacitors are hard to build in IC. Anyway, I am using 3Q, 2C and 1L booster circuit in an IC, but I don't need high power.

Hi,

yes I need at least 500-700 mA for approx 1 ms long flashes, driving a chain of LEDs with a total forward voltage of maybe around 40-60 V. The circuits I've looked at have had the switching regulation controlled by an IC but then used discrete external inductors, capacitors and MOSFET's for switching the inductor and dimming the load. That setup sounds fine by me. I just have trouble finding a useable application note describing a similar setup - most application notes are for driving single or maybe 2-3 white LEDs at a couple of dozen mA (obviously since white LEDs are a major booming market!), or they are buck regulators where the input voltage already is 40-60 V.

I wonder if a switching integrated regulator with a switch frequency of say 700 kHz can ramp up and control such a short flash in a stable manner. I guess it would, since

1 ms would be 700 switching cycles and it shouldn't take that many cycles before it was up enough in voltage over the output capacitor to give the needed current.

Regards,

Bjorn

You would be driving several amps into the transistors. Definitely TO-220 or even TO-3.

I have often needed a bucking LED driver instead. We have the typical application in which a microcontroller controls a set of parallel LEDs in a 7-segment 3 digit display.

The typical design has a lot of power loss in the resistors in series with the LEDs since we have to take VF and supply tolerance into consideration. What I would like to have is a chip that controls the

3digit 7-segment and that controls a buck converter to set the total current in the LEDs. So if x LEDs are on, the current control is set to x current.

Anybody know if such a device exists? It would boost the efficiency and discard of all the resistors too.

Regards

Klaus

I ran some simulations. Using 10mH inductor and 10uF capacitor, it would take 500 ms to ram from 12V to 50V. I use base current of 8 mA on a NPN power transistor and no output load.

Depending on your duty cycle, you would need to drive the transistor harder.

Why would you use such an enormous inductor?

Depending on the switching frequency, I would say something like 1uF cap and 100uH inductor. That would allow the current to settle pretty fast

Regards

Klaus

For enormous power!

He need to store enough charges for 700mA at 50V. 100uH would take forever to charge. My simulation model needs 500ms to pump with 10mH.

You might think that at first, but remember--the stored energy is (1/2)*L*i^2.

For a given peak current a smaller inductor value stores less energy, but it can be charged proportionally faster, so you just increase the switching frequency proportionally and there's no difference in energy delivered.

What a lower value _does_ give you is many fewer turns on your inductor, allowing fatter wire. Fatter wire cuts losses.

More importantly, fewer turns means you can push more current without saturating your core, allowing you to increase the i^2 term a bunch.

So, smaller inductors allow one to *increase* power.

Cheers, James Arthur

If the circuit is 100% efficient.

I do not think, just simulate. Using 2N2219 oscilator, 2N3055 driver,

1K R(collector), 6V VCC. 1uF forward cap, 0.1uF feedback cap and 10uF input cap.

Assume 5K load (50V,10mA):

(10mH, 14V in 300ms) (1mH, 11V in 300ms) (100uH, 7V in 300ms)

Notice that graph3 (100uH) is leveling off.

(1mH, 11V in 300ms)

7V in 300ms)

Adjust the frequencies so that the inductor in each circuit sees the same peak current...

Cheers, James Arthur

300ms) 11V in 300ms) 7V in 300ms)

if the 2N2219 does not saturate or stop oscillating.

I tried 2N2904, 2N2222, 2N2219 and 2N3055, using actual spice3 models. They won't work with a small inductor.

I will work on a spice3 front end (allowing you to select the simulation components) to prove it. I have to prove it to my customer anyway.

On Jul 23, 3:13 pm, linnix wrote:

h2.jpg(1mH, 11V in 300ms)

7V in 300ms)

I should've proposed a driver too, since duty cycle matters, not just frequency, and your 2n3055 isn't going to like being driven 100x faster!

Now this hysteretic converter is complete crap right off the top of my head--so don't go putting it in any airplanes--but it'll switch fast enough to illustrate the point at hand:

=3D=3D=3D=3D=3D=3D=3D .-.-.-. L1 D1 | | | | 100uH MBRS360

+12 >-+-----------+------' '------+------|>|----+--------+->Vout | | R4 | | | | .-----------+--[100k]--+----+ | ,---' | | | | | | --- C2 / \ ZD1 R1 [1k] | R3 [220] '----||----' | --- 1uF --- 30v | | | C1 470pF |/ | | =2E----+ |R2 +-----------------| | | | | [1k] | |>. Q3 =3D=3D=3D =3D=3D=3D | |/ | |/ | ZTX1048A GND GND '--+-----o-----| | |>. | |>. Q2 | Q1 | --- | 2n2222 | 2n2222| / \ =3D=3D=3D | | --- GND | | | D2 | | =3D=3D=3D 1n4148 | | GND | '-------------------------------+ | | R5 [0.1] | | =3D=3D=3D GND

Cheers, James Arthur

=3D=3D=3D=3D=3D=3D=3D=3D=3D Version 4 SHEET 1 880 680 WIRE -112 -48 -256 -48 WIRE 48 -48 -112 -48 WIRE 416 -48 48 -48 WIRE -112 0 -112 -48 WIRE 48 0 48 -48 WIRE 416 0 416 -48 WIRE -256 16 -256 -48 WIRE 176 32 144 32 WIRE 304 32 256 32 WIRE 416 112 416 80 WIRE 512 112 416 112 WIRE 624 112 576 112 WIRE 768 112 624 112 WIRE -256 128 -256 96 WIRE 144 128 144 32 WIRE 144 128 -32 128 WIRE 192 128 144 128 WIRE 304 128 304 32 WIRE 304 128 256 128 WIRE 416 128 416 112 WIRE 416 128 304 128 WIRE 416 144 416 128 WIRE -32 176 -32 128 WIRE 624 176 624 112 WIRE 768 176 768 112 WIRE 48 192 48 80 WIRE 352 192 48 192 WIRE -112 224 -112 80 WIRE -112 224 -208 224 WIRE -112 256 -112 224 WIRE 48 256 48 192 WIRE 416 288 416 240 WIRE 416 288 256 288 WIRE 624 288 624 240 WIRE 768 288 768 240 WIRE -208 304 -208 224 WIRE -176 304 -208 304 WIRE -32 304 -32 256 WIRE -32 304 -176 304 WIRE -16 304 -32 304 WIRE -32 336 -32 304 WIRE 416 352 416 288 WIRE 48 400 48 352 WIRE -32 416 -32 400 WIRE -112 448 -112 352 WIRE 256 448 256 288 WIRE 256 448 -112 448 WIRE 416 480 416 432 FLAG 48 400 0 FLAG 416 480 0 FLAG -256 128 0 FLAG 624 288 0 FLAG -32 416 0 FLAG 768 288 0 SYMBOL npn -16 256 R0 SYMATTR InstName Q1 SYMATTR Value 2N2222 SYMBOL npn -176 256 R0 SYMATTR InstName Q2 SYMATTR Value 2N2222 SYMBOL npn 352 144 R0 SYMATTR InstName Q3 SYMATTR Value ZTX1048A SYMBOL res 400 336 R0 SYMATTR InstName R5 SYMATTR Value .1 SYMBOL res 32 -16 R0 SYMATTR InstName R3 SYMATTR Value 220 SYMBOL ind 400 -16 R0 SYMATTR InstName L1 SYMATTR Value 100=B5H SYMBOL res -128 -16 R0 SYMATTR InstName R1 SYMATTR Value 1k SYMBOL voltage -256 0 R0 SYMATTR InstName V1 SYMATTR Value 12v SYMBOL schottky 512 128 R270 WINDOW 0 32 32 VTop 0 WINDOW 3 0 32 VBottom 0 SYMATTR InstName D1 SYMATTR Value MBRS360 SYMATTR Description Diode SYMATTR Type diode SYMBOL cap 608 176 R0 SYMATTR InstName C1 SYMATTR Value 10=B5F SYMBOL cap 256 112 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C2 SYMATTR Value 470pF SYMBOL diode -16 400 R180 WINDOW 0 24 72 Left 0 WINDOW 3 24 0 Left 0 SYMATTR InstName D2 SYMATTR Value 1N4148 SYMBOL res 272 16 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R4 SYMATTR Value 100k SYMBOL res -48 160 R0 SYMATTR InstName R2 SYMATTR Value 1k SYMBOL zener 784 240 R180 WINDOW 0 24 72 Left 0 WINDOW 3 24 0 Left 0 SYMATTR InstName D3 SYMATTR Value DFLZ33 SYMATTR Description Diode SYMATTR Type diode TEXT -290 506 Left 0 !.tran 0 100mS 0 1uS

Yes, the ZTX1048A works better, but only if i use 330uH. It boosts from 6V to 30V in 300ms. R9 is the output load of 15mA.

vcc 1 0 6 q1 4 2 0 2n2219 q2 7 5 0 ZTX1048A r1 1 2 2700 r2 1 4 330 r3 1 5 47 r9 8 0 2k c1 1 0 1u ic=0v c2 7 2 1u ic=0v c3 4 5 500n ic=0v c4 8 0 100u ic=0v l1 1 7 330u

PS: I don't know if this would be a problem or not. The ZTX1048A Collector to Emitter break down voltage is only 17.5V at 10mA. How would you model this in spice?

I have only a bit of basic knowledge of Spice due to doing more work that is better done than simulated.

I suspect a reduced C-E breakdown voltage as a function of current (as in higher) is due to "forward bias second breakdown". This is a phenomenon of bipolar transistors where power dissipation capability is reduced as a result of higher C-E voltage, even if such higher C-E voltage is well within the device's C-E forward bias voltage limit.

In my past experience of reading power transistor datasheets, power dissipation capability tends to be impaired by "forward bias second breakdown" when C-E voltage is above something like 30-50 volts (varies somewhat from one device to another). This gets "less bad" when the bipolar transistor in question has "hometaxial" structure rather than "epitaxial", though "hometaxial" has appeared to me less-desirable by being slower and "epitaxial" is more common.

The forward-bias second-breakdown phenomenon also has tolerance for higher instantaneous power dissipation at "offending" C-E voltages if this has to be withstood for pulses (generally a few milliseconds or less, better still if fractional millisecond or several microseconds) at a low duty cycle. Transistors with "forward bias second breakdown" vulnerability do often do well as switchers since in such duty they tend to only need to dissipate high power with high C-E voltage for only microsecond ballpark with low duty cycle, and otherwise do their best at approximating an open or a short.

- Don Klipstein ( snipped-for-privacy@misty.com)

Breakdown of 30 to 50 V is more typical. But the ZTX1048A will breakdown as low as 17.5V, which is not a problem for my need (6V to

15V). However, it will indeed be a problem beyond that. Anyway, I would rather use a 2N2222A with higher breakdown voltage. Unfortunately, neither one is available in SOT-23, so back to the drawing (layout) board. My current board is using BC847B, which peak at 12V in simulations.

Yes, that's absolutely a goof in real life, but of no consequence in LTSpice--it doesn't care. I've also supplied Q3 with 50mA of base drive for a

The 2n2222a in SOT-23 is called a MMBT2222A and is marked '1P' on its case. They're everywhere.

Zetex has a number of higher-voltage high-current transistors e.g.:

but if all you want is 15mA you might as well use the MMBT2222a.

Cheers, James Arthur

e.g.:

Thanks, that will do for my low power board.

For higher power, the ZXT1053AK (4A) works nicely with 470uH inductor. There are something in the Zetex parameters that make them work well with small inductors. I haven't figure it out yet, but the simulations are good.

We want a nice parabolic curve to reach high voltage quickly, so we can shut down the circuit ASAP. For 25mA load, The ZXT1053AK 470uH reaches 50V in 100ms and 60V in 300ms.